The present invention relates generally to switching optical path lines in an optical communication system and, more particularly, to an ultrafast, high-sensitivity, waveguide, all-optical switch, made from single-walled carbon nanotube (SWNT) polymer composites, having a switching speed of less than 1 picosecond (ps) for light with a wavelength of about 1.55 micrometers (xcexcm).
The continued development of optical communications requires fast information processing. Therefore, ultrafast, all-optical systems for basic processing at both ends of an optical transmission line are replacing electronic systems. The advantages of all-optical systems include the avoidance of repeated conversions between electrical and optical signals and the faster speed of optical devices over their electronic counterparts.
The reliability and economy of an optical communication network using optical fibers cannot be fully realized by simply connecting two distant components through the optical fibers. Therefore, to further enhance reliability and economy, attempts have been made to improve the availability of the optical fibers by providing an optical switch or switches in the optical fibers. Optical switches function to switch optical information to a standby line, detour around obstacles, or switch optical information to an unused line.
Mechanical optical switches are one type of optical switch used in optical communication systems. Mechanical optical switches function to switch the optical paths by mechanically moving optical components such as the optical fibers. This type of optical switch has a number of drawbacks. The switching speed is low, on the order of millisecond (ms), and the number of switches possible is limited by wear of the parts caused by the mechanical switching operations.
A semiconductor waveguide optical switch is another type of optical switch used in optical communication systems. Semiconductor waveguide optical switches have a switching speed on the order of nanoseconds (ns) and are free from wear. An optical switch having an X-junction optical waveguide shown in FIG. 1 is known in the prior art and is described in U.S. Pat. No. 5,148,505 issued to Yanagawa et al.
As shown in FIG. 1, thin semiconductor layers of a predetermined composition are sequentially laminated as a lower clad layer, a core layer, and an upper clad layer on a semiconductor substrate 51 to form optical waveguides 52 and 53 in a ridge configuration. Optical waveguides 52 and 53 intersect each other in the shape of the letter xe2x80x9cX,xe2x80x9d with a branch angle of xcex8 degrees, to form a junction point or branch point 54. The entire surface of the structure is covered with a thin insulation film. An electrode 55 is used to inject a current of a predetermined value to the optical waveguides which intersect at the branch point 54.
Portions 52a and 53a of the optical waveguides 52 and 53 which lie on one side of the optical waveguides with respect to the branch point 54 constitute input ports, respectively, and the portions 52b and 53b on the opposite side of the branch point 54 constitute output ports, respectively. With the optical switch of the construction shown in FIG. 1, when a predetermined amount of current is injected via the electrode 55, the refractive index of that portion of the core layer into which the current is injected is lowered by the action of the injected carriers. As a result, light waves incident on the input port 53a are subjected to total reflection at the interface between the current injection area and the non-injection area and then transmitted from the output port 52b to the exterior. On the other hand, when no current is injected via the electrode 55, light waves incident on the input port 53a pass straight through the branch point 54 and are transmitted from the output port 53b to the exterior.
Thus, the light waves incident on the input port 53a are transmitted from the output port 52b or 53b depending on whether a current is injected via the electrode 55 or not. In this way, the optical switch of FIG. 1 performs the switching operation.
A branching interference-type modulator shown in FIG. 2 is known as another example of an optical switch. The modulator combines Y-junction optical waveguides of the type shown in FIG. 3. As shown in FIG. 3, each of the Y-junction optical waveguides is constructed by sequentially laminating thin semiconductor layers of a predetermined composition as a lower clad layer, a core layer, and an upper clad layer on a semiconductor substrate 61 to form an optical waveguide 62. The optical waveguide 62 includes a main optical waveguide 62a as an input port for light waves and two output optical waveguides 62b and 62c branching from the main optical waveguide 62 at a predetermined branch angle (xcex8).
Assume that the cross sections of the main input optical waveguide 62a and the output optical waveguides 62b and 62c are the same. Then, the light waves incident on the main input optical waveguide 62a are transmitted outwardly from the output optical waveguides 62b and 62c as light waves of the equal light outputs. More specifically, the light waves of the light output xe2x80x9c2xe2x80x9d incident on the main input optical waveguide 62a are equally divided and then transmitted out from the output optical waveguides 62b and 62c as light waves of light output xe2x80x9c1.xe2x80x9d
As shown in FIG. 2, the output optical waveguides 62b and 62c of one Y-junction optical waveguide are respectively connected to the input optical waveguides 62bxe2x80x2 and 62cxe2x80x2 of the other Y-junction optical waveguide. Electrodes 63a and 63b are respectively formed on the connecting portions of the waveguides. A predetermined voltage can be applied to the electrodes 63a and 63b. With the modulator, light waves incident on the main input optical waveguide 62a are equally divided by the output optical waveguides 62b and 62c. In this case, for example, because the guided light propagating from the output optical waveguide 62c to the optical waveguide 62cxe2x80x2 is subjected to the phase shift according to the voltage applied via the electrode 63a, the guided light is combined or interfered with the guided light propagating from the output optical waveguide 62b to the optical waveguide 62bxe2x80x2. As a result, the light output of the light wave transmitted from the main optical waveguide 62axe2x80x2 varies according to the phase difference between the guided light propagating through the optical waveguide path 62c-62cxe2x80x2 and the guided light propagating through the optical waveguide path 62b-62bxe2x80x2. 
In the case of the branching interference type-modulator, the mode interference of the light waves propagating through the optical paths is used. For this reason, the light output of the light waves to be transmitted depends on the polarization and wavelength of the light waves to be propagated. Accordingly, this type of modulator can be properly operated only for the guided light of a specified polarization and a specified wavelength.
Ultrafast, all-optical switches are crucial for future, high-bit-rate, time-division-multiplexing optical communication systems or free-space, optical-digital, computing systems. So far, many types of ultrafast, all-optical switches have been studied and demonstrated using optical nonlinearities in optical fibers and semiconductor materials. Such devices must satisfy several requirements. They must be compact and, in this respect, semiconductor devices are preferable to optical fiber devices. In addition, they should be independent of polarization and, in this respect, normal-incident devices are preferable to waveguide devices. In the normal-incident devices, however, large optical nonlinearities are needed because the interaction length is small.
The nonlinear optical properties of active semiconductor waveguides have proven to be of great interest in terms of their possible application to high-speed devices, including subpicosecond optical switches. In semiconductor optical devices, the optical nonlinearity usually comes from the excitation of real or virtual carriers by a strong control (pump) beam. Much work has been directed to ultrafast and low-energy switching in a current injected GaAs/AlGaAs nonlinear material. See, e.g., J. Paye and D. Hulin, xe2x80x9cMonochromic all-optic gate with 1 ps response time,xe2x80x9d Appl. Phys. Lett. 62, 1326 (1993); S. Lee, B. McGinnis, R. Jin, J. Yumoto, G. Khitrova, H. Gibbs, R. Binder, S. Koch, and N. Peyghambarian, xe2x80x9cSubpicosecond switchingin a current injected GaAs/AlGaAs multiple-quantum-well nonlineardirectional coupler,xe2x80x9d Appl. Phys. Lett. 64, 454 (1994); and R. Takahashi, Y. Kawamura, and H. Iwamura, xe2x80x9cUltrafast 1.55 xcexcm all-optical switching using low-temperature-grown multiple quantum wells,xe2x80x9d Appl. Phys. Lett. 68, 153 (1996). The GaAs-based material is expensive and cannot be integrated into silicon-incorporating structures.
Although the generation of real carriers can induce a large optical nonlinearity in semiconductor devices, the device performance is limited by the long (nanoseconds) recovery time. Off-resonance excitation in the optical Stark effect regime (which is a coupling between the electric field of light and the energy levels of the material) has a subpicosecond response, but it requires a very high switching intensity (typically, a few Gigawatts/cm2) and suffers from an accompanying long-lasting recovery component due to the generation of real carriers by one- or two-photon absorption. In both cases, large loss is another limiting factor because the wavelength of light must be close to the semiconductor absorption-band edge in order to achieve a large optical nonlinearity.
Due to the limits imposed by the properties of the materials used for the existing all-optical switches, the fastest speed of all-optical switches is around 1 picosecond at the optical communication wavelength of about 1.55 xcexcm. In addition, the fabrication process of some existing ultrafast optical switches is not compatible with the current silicon technology. Therefore, if one can find a faster nonlinear material with similar structure but easy to incorporate into the integrated system, one can improve the speed of the optical communication as well as reduce the price of the device.
To overcome the shortcomings of existing all-optical switches, a new all-optical switch is provided. An object of the present invention is to provide an improved all-optical switch made of a faster nonlinear material with similar structure to existing materials. It is still another object of the present invention to provide an optical switch in which the switching operation is not mechanically affected and, therefore, wear is not caused by the switching operation and the switching speed is high.
A related object is to provide a third-order nonlinear optical material having unique properties and a process for production of such material. More specifically, it is an object of the present invention to provide a material which excels in processability and has a higher relaxation speed as compared with conventional materials. A further object of this invention is to provide a third-order nonlinear optical material satisfying certain requirements, namely: (1) large third-order nonlinear susceptibility; (2) high transparency (small absorption) in the operating optical communication wavelength range of about 1.55 xcexcm; and (3) high relaxation speed. Another object of the present invention is to provide a material for an improved all-optical switch that can be incorporated easily into integrated systems and, especially, is compatible with silicon-incorporating structures. Another object of the present invention is to provide a material for an improved all-optical switch that is cost-competitive with existing switch materials. Still another object of the present invention is to provide a third-order nonlinear optical material comprising readily available raw material components, preventing restrictions on manufacture.
To achieve these and other objects, and in view of its purposes, the present invention provides an ultrafast all-optical nonlinear switch. The switch has as components a substrate and a material disposed on the substrate. In one embodiment, the material includes a plurality of single-walled carbon nanotubes and a polymer forming a composite. Preferably, the polymer is polyimide. In another embodiment, the material includes a plurality of single-walled carbon nanotubes incorporated into a silica. The nanotube loading in the material is less than about 0.1 wt %. The material is a substantially transparent, third-order nonlinear optical material. The switch has a switching speed of less than 1 picosecond for light with a wavelength of about 1.55 micrometers. Also disclosed is a process for preparing the ultrafast all-optical nonlinear switch.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.